The present invention relates generally to interferometers, and more particularly, to an apparatus and method for providing unambiguous angle measurements simultaneously in azimuth and elevation in such interferometers using a minimum number of antenna elements.
Prior art in the area of resolving unambiguous angle information in interferometers may be found with reference to the following patents. U.S. Pat. Nos. 3,697,997 and 3,740,002 show simple two antenna (for each axis) interferometers. They are incapable of resolving angle ambiguities when antenna elements are farther apart than 1/2 wavelength of the received signal. This severely limits their angle accuracy since angle accuracy improves with antenna separation.
U.S. Pat. No. 4,170,774 uses amplitude to resolve the angular ambiguities associated with a phase interferometer. To obtain good amplitude performance requires high antenna gain and a corresponding reduction in the interferometer's field of view. Also, the antenna elements must be squinted away from each other which reduces the accuracy of the phase interferometer since the antenna elements must operate further off their boresight to cover a given field of view.
U.S. Pat. No. 4,540,139 addresses the problem of detecting any polarization with an interferometer using linearly polarized receiving elements. Most current systems that detect signals with various polarizations use circularly polarized antenna elements. This patent uses two antenna elements per baseline and is incapable of resolving angular ambiguities for long antenna element separations. This limits its angular accuracy as noted above for U.S. Pat. Nos. 3,697,997 and 3,740,002. U.S. Pat. Nos. 4,638,320 and 4,639,733 are interferometer systems using more than two antenna elements per spatial angle to achieve high angular resolution and resolve angular ambiguities.
Wide frequency coverage interferometers require broadband antenna elements whose diameter is determined by the lowest frequency that must be received. The size of the antenna element limits the center to center spacings of the elements to approximately 1/2 a wavelength at the lowest frequency of interest. Thus, at higher frequencies of interest, the interferometer based on two closely spaced antenna elements is ambiguous in angle. The present invention is adapted to resolve these ambiguities.
Thus it is an objective of the present invention to provide for apparatus and methods that provides for simultaneous unambiguous angle measurements in azimuth and elevation in an interferometer using a minimum number of antenna elements.